1914] Grinnell: Mammals and Birds of the Colorado Valh y 245 



sandy soil along ravine bottoms or along washes cutting the elevated 

 mesas. This closer associational phase of occurrence is significant on 

 comparison with the local distribution of Perognathus spinal us. For- 

 mosus and spinatus both live west of the river, and both occupy the 

 same general associations (see figs. C, F). But locally the two are 

 often found on separate ground, spinatus adhering most closely to 

 the vicinity of boulder-strewn hillsides and fractured outcrops, while 

 formosus prefers smoother ground, as noted above. 



At Riverside Mountain formosus was caught on the mesa, but not 

 next to the abruptly rising base of the mountain where spinatus was 

 found. But opposite Cibola, where pocket mice were phenomenally 

 abundant, examples of Perognathus spinatus, P. formosus and P. peni- 

 cillatus were all caught within a radius of fifty feet. The first two 

 species were found in a single trap on successive mornings. 



As further considerations with regard to the occurrence of for- 

 mosus and spinatus on common ground, it is to be remembered that 

 the two* species belong to separate subgenera (Perognathus and Chae- 

 todipus, respectively) , and thus have structures so different that impor- 

 tant differences in food or other requirements are suggested; so that 

 the forms are probably complementary instead of violently competitive. 



The breeding time of Perognathus formosus is indicated by the 

 capture of blue-pelaged young at Potholes and Pilot Knob on April 

 28 and 29, and May 10 to 14. Preceding the first-named date no 

 signs of breeding were in evidence, so that relative lateness in this 

 annual function is indicated. 



Perognathus penicillatus penicillatus Woodhouse 



Colorado Desert Pocket Mouse 

 » 

 A series of 196 specimens preserved, 184 as skins-with-skulls (nos. 



9692-9S75), two as skeletons (nos. 10720, 10721), and ten as alco- 

 holics (nos. 10798-10807). The following localities are represented: 

 California side : five miles below Needles, 8 ; opposite The Needles, 10 ; 

 Chemehuevis Valley, 17; near Riverside Mountain, 16; near Blythe, 

 2 ; above Ehrenberg, 1 ; opposite Cibola, 35 ; twenty miles above Pic- 

 acho, 10 ; eight miles below Picacho, 8 ; Potholes, 1 ; four miles below 

 Potholes, 1; five miles northeast of Yuma, 5; near Pilot Knob, 8. 

 Arizona side: Mellen, 12; above Bill Williams River, 2; Ehrenberg, 

 16 ; twenty-five miles below Ehrenberg, 3 ; ten miles below Cibola. 27 ; 

 five miles northeast of Laguna, 8 ; near Yuma, 6. 



